Device for the control of a circuit for the vertical deflection of a spot scanning a screen

ABSTRACT

Disclosed is a device to control a circuit for the vertical deflection of a spot scanning a screen, and more particularly a control device whose output amplifier stage works in class D mode at the rate of a switching signal called a first switching signal. The control device has an internal auxiliary supply to generate the overvoltage needed for the fast flyback of the spot. This auxiliary power supply is a switching voltage generation circuit whose switching signal, called a second switching signal, is synchronous with the first switching signal. The present invention has been shown to used advantageously in television screens and/or computer screens.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is based upon and claims priority from prior FrenchPatent Application No. 0101839, filed Feb. 7, 2001, the disclosure ofwhich is hereby incorporated by reference in its entirety.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention generally relates to the field of devices for thecontrol of a circuit for the deflection of a spot on a screen and moreparticularly the present invention relates to a circuit for the verticaldeflection of a spot scanning on a screen, especially for televisionscreens and computer screens using class D amplifiers.

2. Description of Related Art

Generally, the spot scans the screen in two orthogonal directions,typically a horizontal direction and a vertical direction. Horizontal orline scanning consists of shifting the spot from left to right and thenmaking a line return at the end of the line to go to the next line.Simultaneously, in vertical scanning, the screen is scanned from top tobottom. When the spot reaches the bottom of the screen, a flyback isdone to bring the spot to the top of the screen to begin the next scan.The vertical deflection of the spot therefore comprises an outbound orforward phase of vertical scanning (from top to bottom) and a phase forthe fast flyback of the spot. This vertical deflection of the spot ispreformed by placing the spot in a magnetic field created by one or morevertical deflection coils as part of a vertical deflection controlcircuit.

One device used in the control circuits for the vertical deflection of aspot scan on a screen is class AB (linear) or class D amplifiers. ClassAB amplifiers are amplifiers whose output stage comprises transistorsworking in their linear zone and producing a quasi-DC voltage for thispurpose. Class D amplifiers are amplifiers whose output stage comprisestransistors working alternately in their saturation zone and in theircut-off zone so as to generate a “square-wave” voltage (an alternationof high and low levels). The high frequency components of this“square-wave” voltage are then filtered to obtain the mean value of thevoltage.

The role of these amplifiers is to amplify an instructed-value signal,generally a sawtoothed signal, in order to deliver a vertical scanningcontrol signal as shown in FIG. 1. The amplifier delivers a linearvoltage ramp (the voltage decreases from +V_(M) to −V_(M)) during theforward phase of the vertical scanning and delivers an overvoltage VF ofseveral tens of volts (preferably 70 to 75 volts) during the spotflyback phase to ensure a fast spot flyback.

One control for a class D amplifier is described in a verticaldeflection control circuit in the French patent application No. 9900715,with inventor Philippe Maige and commonly assigned withSTMicroelectronics SA Société anonyme, and incorporated by reference inits entirety. FIGS. 2, 3 and 4 of the present invention are taken fromthe French patent application 9900715. FIG. 2 illustrate a diagram ofthe architecture of a vertical deflection circuit control device workingas a class D device and FIGS. 3 and 4 illustrate two possibleembodiments of the output stage of this device. Referring to FIG. 2, thereference control device DCS gives a vertical scanning signal to thevertical deflection circuit referenced DV. The control device DCS has apre-amplification stage PMP mounted as an inverter, an output stage BLS,control means PWM and detection means DTF to control the output stage,and a smoothing filter F. The pre-amplification stage PMP consists of anoperational amplifier AO mounted as an inverter whose inverter input isconnected to the input terminal BE of the control device by means of aresistor R1. This inverter input is also connected to the output of theoperational amplifier AO by means of a resistor R4 and secondly to aterminal BA of the vertical deflection circuit by means of a resistorR2. A low-value resistor R3 grounds the terminal BA. The resistors R1and R4 fix the gain of the pre-amplification stage and the resistors R2and R4 fix the gain of the feedback loop. A reference voltage Ref whichmay be zero, is applied to the non-inverter input of the operationalamplifier.

The operational amplifier AO delivers an error signal which is used bythe control means PWM and the detection means DTF to control the outputstage BLS. Referring to FIG. 3, the output stage BLS comprises anamplifier stage ETS formed by two NMOS type transistors T1 and T2series-connected between the node N1 and the negative terminal −Vcc ofthe main power supply of the control device. The common terminal ofthese two transistors is the output terminal BSS of the amplifier stageETS. A diode D1 (D2 respectively) is mounted in an anti-parallelconnection at the terminals of the transistor T1 (T2 respectively). Thatis, the anode of the diode D1 (D2 respectively) is connected to thesource of the transistor T1 (T2 respectively) and the cathode of thediode D1 (D2 respectively) is connected to the drain of the transistorT1 (T2 respectively). The gates of the transistors T1 and T2 arecontrolled by opposite signals coming from the control means PWM.Hereinafter in the description, the control signal applied to the gateof the transistor T1 is called a switching signal COM_(T1). The outputterminal BSS of the amplifier stage ETS is connected to the outputterminal BS of the control device by means of the smoothing filter Fformed by an inductance coil and a capacitor.

The output stage BLS also has means MFB designed to control thegeneration of the overvoltage needed for the fast spot flyback.Referring to FIG. 3, the means MFB comprise a first two-way switchformed by an NMOS transistor T3 whose source is connected to the drainof the transistor T1 and whose drain is connected to a terminal BALdesigned to receive a voltage VF delivered by an auxiliary power supplyALM. The means MFB also comprise a second two-way switch formed by anNMOS transistor T4 whose drain is connected to the drain of thetransistor T1 and whose source is connected to the positive terminal+Vcc of the main power supply. A diode D3 (D4 respectively) is mountedin an anti-parallel connection at the terminals of the transistor T3 (T4respectively). The transistors T3 and T4 are also controlled by thedetection means DTF.

This control device works as follows. During the forward phase ofvertical scanning, the control means PWM deliver control pulses(switching signal COM_(T1)) to make the transistors T1 and T2alternately conductive and non-conductive with a very high switchingfrequency, in the range of 100 KHz. During this phase, the transistor T4is conductive and the transistor T3 is off.

During this phase, the duration of the control pulses, designed to makethe transistor T1 conductive, decreases from about 90% of the period ofthe switching signal to about 10% of the period of the switching signal.Conversely, the duration of the control pulses, designed to make thetransistor T2 conductive, increases from about 10% of the period of theswitching signal to about 90% of the period of the switching signal. Thevoltage at the terminal BSS is filtered by the filter F to keep only themean value.

It is important to note that, when the output voltage of the controldevice is zero, the control pulses of the switching signal take up about50% of the switching period.

During the fast flyback phase of the spot, the control means PWMrespectively make the transistors T1 and T2 conductive andnon-conductive and the detection means DTF respectively make thetransistors T3 and T4 conductive and non-conductive. The voltage VFgiven by the auxiliary power supply ALM is then delivered to the outputterminal BS of the control device.

At present, the auxiliary power supply ALM used to generate theovervoltage is either external, i.e., external to the control device orinternal, i.e., integrated into the control device. In the former casecorresponding to the case shown in FIG. 3, the auxiliary power supply isgenerally a voltage step-up converter which, from a power supply voltageof about 24 volts, gives an auxiliary power supply voltage of about 70volts. This approach is very satisfactory because the control devicerequires the presence of an external auxiliary supply in order tofunction.

The case of an auxiliary supply known as an internal supply isillustrated in FIG. 4. The internal auxiliary supply is formed by achemical capacitor C whose first terminal is connected for example tothe positive terminal +Vcc of the main power supply by means of thefirst two-way switch T4 and to the drain of the transistor T1. Thesecond switch T3 is then used to connect the second terminal of thecapacitor to the positive terminal +Vcc of the main power supply. Thesecond terminal of the capacitor is also connected to the negativeterminal −Vcc of the main power supply by means of a third two-wayswitch T6. The capacitor C is then charged by the main power supplyvoltage during the forward phase of the vertical scanning and is placedin series with the positive terminal of the main power supply during thefast spot flyback phase to generate the overvoltage. In this circuit,the voltage values of the main supply and of the auxiliary supply arecorrelated. This approach although useful, is not without its drawbacks.One drawback is that the voltage generated by this internal circuit isnot sufficient. Typically the voltage generated is in the range of 36 Vfor a main power supply of 24 V. One solution to this drawback consistsof increasing the value of the main power supply voltage. However, thisapproach of increased power supply voltage has its drawbacks. Forexample one drawback is it requires an increase in the heat dissipationin the control device. This goes against the goal desired through theuse of class D amplifiers. Accordingly, a need exists to provide anauxiliary power supply to overcome these shortcomings for the control ofvertical deflection circuits.

SUMMARY OF THE INVENTION

The present invention results from research carried out on devices forthe control of vertical deflection circuits with an output stageworking. The present invention has been shown to be used advantageouslywith an output stage working in class D mode. Moreover, the presentinvention provides a control device that does not require any auxiliaryexternal power supply and is capable of producing a high voltage valuein the range of 70 to 75 volts without disturbing the small-signaloperation of the amplifier of the control device, especially during theforward scanning stage.

The present invention is a device to control a circuit for the verticaldeflection of a spot scanning a screen, comprising an output amplifierstage powered by a main power supply for the vertical scanning controland an auxiliary power supply capable of generating an overvoltage atthe vertical deflection circuit for the fast flyback of the spot, theoutput amplifier stage working in class D mode at the rate of aswitching signal called the first switching signal, the auxiliary powersupply being also internal to the control device, wherein the auxiliarypower supply is a switching voltage generation circuit whose switchingsignal, called a second switching signal, is synchronous with the firstswitching signal.

According to a particular embodiment, the switching voltage generationcircuit comprises an output terminal to deliver the overvoltage, aninductive element series-mounted with a switch between a first terminalof the main power supply and a second terminal of the main power supply,a capacitive element connected between the output terminal and thesecond terminal of the main power supply, a diode whose anode isconnected to an intermediate point between the inductive element and theswitch and whose cathode is connected to the output terminal and aswitch control circuit to deliver the second switching signal designedto control the state of the switch, the second switching signal beingsynchronous with the first switching signal.

In this embodiment for example, the switch control circuit is preferablya DC/DC regulator receiving at input a reference voltage signal, asignal representing the voltage present at the output terminal of theswitching voltage generating circuit, a clock signal synchronous withthe first switching signal and a ramp signal used to modulate theduration of the control pulses of the second switching signal.

BRIEF DESCRIPTION OF THE DRAWINGS

The subject matter which is regarded as the invention is particularlypointed out and distinctly claimed in the claims at the conclusion ofthe specification. The foregoing and other features, and advantages ofthe invention will be apparent from the following detailed descriptiontaken in conjunction with the accompanying drawings.

FIG. 1, already described, represents the vertical scanning controlsignal delivered by a vertical deflection control device.

FIG. 2, already described, represents the drawing of a prior art controldevice for creating the vertical scanning control signal of FIG. 1.

FIGS. 3 and 4, already described, represent two different embodiments ofthe output stage of the control device of FIG. 2.

FIG. 5 is a drawing of a switching voltage generation circuit capable ofgenerating a overvoltage, which is integrated into the control device,according to the present invention.

FIG. 6 is an waveform timing illustration of the switching signals ofthe amplifier of the control device and of the switching voltagegeneration circuit as well as the form of the current in the inductivecoil of the switching voltage generation circuit during the forwardstage of vertical scanning, according to the present invention.

DETAILED DESCRIPTION OF AN EMBODIMENT

It should be understood that these embodiments are only examples of themany advantageous uses of the innovative teachings herein. In general,statements made in the specification of the present application do notnecessarily limit any of the various claimed inventions. Moreover, somestatements may apply to some inventive features but not to others. Ingeneral, unless otherwise indicated, singular elements maybe in theplural and vice versa with no loss of generality.

In the drawing like numerals refer to like parts through several views.

The invention provides for the integration into the control device of aswitching voltage generation circuit that generates the overvoltage fromvoltage given by the main power supply of the control device. In oneembodiment, the switching signal of the switching voltage generationcircuit and the switching signal COM_(T1) of the output amplifier stageBLS are synchronous so that the switching signal of the switchingvoltage generation circuit does not disturb the working of the outputamplifier stage ETS and vice versa.

A possible structure of the switching voltage generation circuit ALM isshown in FIG. 5. This device ALM is integrated into the control deviceDCS. In the present case, it is a voltage step-up converter. Itcomprises an inductance coil L, a first terminal of which is connectedto the positive terminal +Vcc of the main power supply of the controldevice, and the second terminal of which is connected to the firstterminal of a resistor R5 by means of a switch herein formed by a NMOStype transistor T5. The drain of the transistor T5 is connected to thesecond terminal of the inductance coil L and its source is connected tothe first resistor R5. The second terminal of the resistor R5 isconnected to the negative terminal −Vcc of the main power supply. Thevoltage generation device ALM furthermore comprises a diode D5 whoseanode is connected to the second terminal of the inductance coil L andwhose cathode is connected to an output terminal S. A capacitor Cf isconnected between the output terminal S and the negative terminal −Vccof the main power supply. During the forward phase of vertical scanning,this capacitor is charged with the current coming from the inductancecoil L so as to reach an overvoltage value VF of about 70 volts.

To control the transistor T5 and regulate the voltage at the outputterminal S of the device ALM, a DC/DC regulator referenced Reg isprovided. This regulator delivers a switching signal COM_(T5) to controlthe state of conduction of the transistor T5. The switching signalCOM_(T5) is given to the gate of the transistor T5. In the example shownin FIG. 5, the architecture of the regulator is that of the regulatorUC3842 commercially distributed inter alia by the firm STMicroelectronics. The regulator Reg receives a reference DC voltagesignal Vref at input, a signal SVF representing the value of the voltageVF at the output terminal S, a clock signal CK and a ramp signal RP. Thesignal SVF herein corresponds to a voltage at the midpoint of a dividerbridge formed by two resistors R6 and R7 series-mounted between theoutput terminal S and the negative terminal −Vcc of the main supply. Theramp signal RP is the voltage signal present at the source of thetransistor T5, the resistor R5 herein playing a role of acurrent/voltage converter. Thus, the regulator Reg generates a controlpulse at each pulse of the clock signal and the duration of this controlpulse is modulated as a function of the value of the signals SVF and RP.

The working of a switching voltage generation circuit such as this iswell known to those skilled in the art. This device works in such a waythat, when the transistor T5 is conductive, the inductance coil L storesthe energy and the current i_(L) flowing through it increases and whenthe transistor T5 is off, the inductance coil L restores the storedenergy and the capacitor Cf gets charged.

We shall now give a more detailed description of the working of thisvoltage generation device as compared with that of the control device ofthe invention with reference more particularly to FIGS. 5 and 6. FIG. 6shows a waveform of the signals COM_(T1), COM_(T5) and i_(L) at thebeginning, middle and end of the forward phase of vertical scanning.These three periods of the forward phase of vertical scanning areseparated in the figure by thin lines of dots and dashes.

During the vertical scanning forward phase, the capacitor Cf getscharged. As shown in FIG. 5, as and when the capacitor Cf gets charged,the demand for current by this capacitor diminishes and the cyclicalratio of the switching signal COM_(T5) then diminishes. The leadingedges of the switching signal COM_(T5) and of the switching signalCOM_(T1) are synchronous. Advantageously, the capacitor Cf gets chargedduring the first half of the vertical scanning forward phase until itreaches the value VF and this charge is held in the capacitor during thesecond half of this vertical scanning forward phase. The cyclical ratioof the switching signal COM_(T5) is therefore very low during thissecond half of the stage and is just enough to compensate for theJoule's heat losses in the resistive divider bridge.

Preferably, it is seen to it that the duration of each control pulse ofthe switching signal COM_(T5) is lower than that of the correspondingcontrol pulse of the switching signal COM_(T1) so that the trailingedges of the switching signal COM_(T5) do not disturb the working of theoutput amplifier stage ETS of the control device DCS.

During the fast flyback phase of the spot, the switching signal COM_(T1)remains at a high level while the switching signal COM_(T5) remains atthe low level.

It must be noted that this could also be integrated into an AB classcontrol device.

Although a specific embodiment of the invention has been disclosed, itwill be understood by those having skill in the art that changes can bemade to this specific embodiment without departing from the spirit andscope of the invention. The scope of the invention is not to berestricted, therefore, to the specific embodiment, and it is intendedthat the appended claims cover any and all such applications,modifications, and embodiments within the scope of the presentinvention.

What is claimed is:
 1. A device to control a circuit for the verticaldeflection of a spot scanning a screen, the device comprising: an outputamplifier stage receiving power from a main power supply for thevertical scanning control; and an auxiliary power supply for generatingan overvoltage at the vertical deflection circuit for the fast flybackof the spot; wherein the output amplifier stage operates at a rate of aswitching signal called a first switching signal; wherein the auxiliarypower supply is a switching voltage generation circuit whose switchingsignal, called a second switching signal, is synchronous with the firstswitching signal.
 2. The device according to claim 1, wherein the outputamplifier is working in a class D mode.
 3. The device according to claim1, wherein the auxiliary power supply includes a switching voltagegeneration circuit comprising: an output terminal to deliver the overvoltage; an inductive element series-mounted with a switch between afirst terminal of the main power supply and a second terminal of themain power supply; a capacitive element connected between the outputterminal and the second terminal of the main power supply; and a diodewhose anode is connected to an intermediate point between the inductiveelement and the switch and whose cathode is connected to the outputterminal and a switch control circuit to deliver the second switchingsignal designed to control the state of the switch, the second switchingsignal being synchronous with the first switching signal.
 4. The deviceaccording to claim 3, wherein the switch is an NMOS type transistor. 5.The device according to claim 3, wherein the switch control circuitfurther comprises a DC/DC regulator having: a reference voltage inputterminal for receiving a reference voltage signal; a clock inputterminal for receiving clock signal synchronous with the first switchingsignal; a ramp input terminal for receiving a ramp signal; an inputterminal for receiving a signal representing the voltage present at theoutput terminal of the switching voltage generating circuit; and anoutput terminal for providing the second switching signal.
 6. The deviceaccording to claim 4, wherein the switch control circuit furthercomprises a DC/DC regulator: a reference voltage input terminal forreceiving a reference voltage signal; a clock input terminal forreceiving clock signal synchronous with the first switching signal; aramp input terminal for receiving a ramp signal; an input terminal forreceiving a signal representing the voltage present at the outputterminal of the switching voltage generating circuit; and an outputterminal for providing the second switching signal.
 7. The deviceaccording to claim 5, wherein the switching voltage generation circuitcomprises: a resistive divider bridge formed by two resistorsseries-connected between the output terminal and the second terminal ofthe main power supply; wherein the signal representing the voltagepresent at the output terminal of the switching voltage generationcircuit corresponds to a voltage at a midpoint of the resistive dividerbridge.
 8. The device according to claim 6, wherein the switchingvoltage generation circuit comprises: a resistive divider bridge formedby two resistors series-connected between the output terminal and thesecond terminal of the main power supply; wherein the signalrepresenting the voltage present at the output terminal of the switchingvoltage generation circuit corresponds to the voltage at the midpoint ofthe resistive divider bridge.
 9. The device according to claim 3,wherein the first and second switching signals both have control pulses,wherein the leading edges of the control pulses of the second switchingsignal are synchronous with the leading edges of the control pulses ofthe first switching signal and wherein each control pulse of the secondswitching signal has a duration smaller than the duration of thecorresponding control pulse of the first switching signal.
 10. Thedevice according to claim 7, wherein the first and second switchingsignals both have control pulses, wherein the leading edges of thecontrol pulses of the second switching signal are synchronous with theleading edges of the control pulses of the first switching signal andwherein each control pulse of the second switching signal has a durationsmaller than the duration of the corresponding control pulse of thefirst switching signal.
 11. A device to control a circuit for thedeflection of a spot scanning a screen, the device comprising: an outputamplifier stage receiving power from a main power supply for thescanning control, the output amplifier stage operating at a rate of aswitching signal called a first switching signal; and an auxiliary powersupply formed from a switching voltage generation circuit for generatingan over voltage at the deflection circuit for the fast flyback of thespot, the switching voltage generation circuit comprising: a secondswitching signal which is synchronous with the first switching signal;an output terminal to deliver the over voltage; an inductive elementseries-mounted with a switch between a first terminal of the main powersupply and a second terminal of the main power supply; a capacitiveelement connected between the output terminal and the second terminal ofthe main power supply; and a diode whose anode is connected to anintermediate point between the inductive element and the switch andwhose cathode is connected to the output terminal and a switch controlcircuit to deliver the second switching signal designed to control thestate of the switch, the second switching signal being synchronous withthe first switching signal.
 12. The device according to claim 11,wherein the switch is an NMOS type transistor.
 13. The device accordingto claim 11, wherein the output amplifier is working in a class D modeas part of a class D amplifier.
 14. The device according to claim 11,wherein the switch control circuit further comprises a DC/DC regulatorhaving: a reference voltage input terminal for receiving a referencevoltage signal; a clock input terminal for receiving clock signalsynchronous with the first switching signal; a ramp input terminal forreceiving a ramp signal; an input terminal for receiving a signalrepresenting the voltage present at the output terminal of the switchingvoltage generating circuit; and an output terminal for providing saidsecond switching signal.
 15. The device according to claim 13, whereinthe switch control circuit is preferably a DC/DC regulator receiving: areference voltage input terminal for receiving a reference voltagesignal; a clock input terminal for receiving clock signal synchronouswith the first switching signal; a ramp input terminal for receiving aramp signal; and an input terminal for receiving a signal representingthe voltage present at the output terminal of the switching voltagegenerating circuit; and an output terminal for providing said secondswitching signal.
 16. The device according to claim 15, wherein theswitching voltage generation circuit comprises: a resistive dividerbridge formed by two resistors series-connected between the outputterminal and the second terminal of the main power supply; wherein thesignal representing the voltage present at the output terminal of theswitching voltage generation circuit corresponds to the voltage at themidpoint of the resistive divider bridge.